Hoppers are commonly used to feed product downwardly into packaging containers. In the case of vertical form-fill-and-seal packaging, the lower end of the hopper can be disposed just above the level at which the successive spaced-apart transverse seals are created in the formed tubular packaging material to separate the accurately metered batches of product fed down one after the other through the hopper. The metering may be by volume, number or weight. In any event, it is important that all the product in each batch entering the top of the hopper constitutes that batch when it enters the partly formed package at the bottom of the hopper, so that at all times, it is correctly metered batches which are being packaged.
Many products are sufficiently fluent in the hopper to reliably pass therethrough under the influence of gravity alone. However, some products are notoriously difficult to package on vertical form-fill-and-seal packaging machines, because the product gets caught in the hopper causing underweight packages to be formed and ultimately cessation of the packaging process while the hopper is cleared of obstructions. Some products not only tend to adhere to the hopper wall(s), but the product making up a batch does not naturally form a compact mass for economical packaging. One product range which has proved difficult to package reliably in vertical form-fill-and-seal machines is fresh vegetable leaves and notably spinach leaves. In a known prior art arrangement, the forming of 500 g packages of fresh spinach leaves cannot be completed at more than 7 to 8 packages a minute, and then only with continuous operator supervision and frequent manual intervention. Further, the 500 g packages produced are some 500 mm long, more than 50% longer than necessary if satisfactory compaction of the leaves could be achieved.